Technical Field
Embodiments of the invention relate generally to boiler tubes, and more specifically, to an apparatus and method for determining the integrity of a boiler tube.
Discussion of Art
Many power generation plants utilize steam generators to power turbines which generate electrical power. Such power plants typically use boilers to generate and transport steam via boiler bank tube assemblies which include one or more tubes, referred to herein as “boiler tubes” and/or simply as “tubes,” that fluidly connect an upper drum to a lower drum. The tubes of such boiler bank tube assemblies are typically fixed to the drums via a process known as expansion wherein the ends of the tubes are inserted into corresponding receiving holes on the drums, which usually have larger diameters than the tubes, and the ends of the tubes are then expanded into the drum via pneumatic expanders and rolling motors so as to form a seal between the drum and the tubes.
During boiler operations, water, and/or another medium, is heated in the tubes and/or drums such that it becomes a gas, e.g., steam, which pressurizes the tubes and drums. As such, the tubes of boiler bank tube assemblies experience thermal stresses, which may result in the emergence of new, and/or the agitation of existing, defects, e.g., thinning and/or cracking of a tube wall. Such defects in turn may degrade the integrity of the tubes, which, as used herein, refers to the likelihood that a tube will rupture and/or leak during boiler operations. For example, a tube with a high integrity is unlikely to rupture and/or leak, while a tube with a low integrity is likely to rupture and/or leak.
Generally, the likelihood that defects will develop or become agitated via thermal stress is dependent on the quality of the expansion process, i.e., the precision and consistency with respect to the shape and thickness of the ends of the tubes expanded and rolled within the drums. Many expansion processes, however, result in tubes being “over rolled,” which as used herein, means the over expansion and/or over use of rolling motors that results in roll marks and/or excessively large internal diameter (“ID”) ridges within the tubes. Both roll marks and/or excessively large ID ridges often form areas that experience high thermal stresses when subjected to the heat and/or pressurization of steam production, and are thus likely to result in cracking of a tube. Such cracks typically fall into two categories: 1) “circumferential cracks,” which as used herein refers to cracks that generally follow the circumference of a tube, i.e., “circular cracks”; and 2) “axial cracks,” which as used herein refers to cracks that generally follow the longitudinal axis of a tube, i.e., “long cracks.”
As stated above, a tube with a low integrity may rupture and/or leak, which in turn may result in costly downtime of a boiler and/or cause severe physical harm and/or death to a bystander. Accordingly, many boiler tubes are regularly inspected to determine their integrity, and are subsequently repaired or replaced if found to have a significant risk of rupturing. Many methods for inspecting the integrity of such tubes, however, suffer from a variety of problems. For example, many inspection methods rely on ultrasonic shear wave technologies which are susceptible to false positives resulting from excessively large ID ridges, roll marks, and/or other geometric formations, that are inherently created in the tube by the expansion process which have not developed into cracks. Mistaking a false positive for an actual defect/crack, in turn, may result in unnecessary down time of a boiler and/or unneeded and costly maintenance.
What is needed, therefore, is an improved apparatus and method for determining the integrity of a tube.